1. Technical Field of the Invention
The present invention relates to vehicle alternators and, more particularly, to a vehicle alternator having a brush unit to supply excitation current to a field winding of a rotor.
2. Description of the Related Art
With vehicle alternators of the related art, it has been a general practice to have a brush unit through which excitation current is supplied to a field winding of a rotor. The brush unit generally includes a brush holder disposed above slip rings formed on a rotary shaft of the rotor, and brushes movably held in the brush holder in sliding contact with the slip rings. Entire circumferences of the brushes are covered with the brush holder and a sealing plate. Due to sliding contact with the brushes and the slip rings, there has been an issue of brush-wear particles scattering from the brushes and accumulating in an area inside the brush holder. With the brush-wear particles accumulated inside the brush holder, an issue arises with deterioration taking place in electric insulation between the associated slip rings or between the associated brushes.
To address such an issue, an attempt has been made to provide a brush unit having a brush holder formed with a brush-wear particle discharge port through which brush-wear particles are discharged to the outside of the brush holder in a manner as disclosed in, for instance, U.S. Pat. No. 6,294,856.
However, the provision of the brush-wear particle discharge port formed in the brush holder causes an increase in probability of water intruding the inside of the brush holder. Especially, if the brush holder is wholly submerged under water, an inevitable issue arises with water intruding the inside of the brush holder. When this takes place, muddy water or salt water adhere onto the brush holder, causing the brushes to be brought out of contact with the slip rings with resultant defective operation of generating electric power.
With the vehicle alternator of the related art, further, it has been a usual practice for electric component parts such as a voltage regulator and a rectifier or the like to be mounted on the vehicle alternator in a rear area thereof at a position remotest from a pulley coupled to the rotary shaft at a front end thereof. These electric component parts are covered with a rear cover configured in a structure to block the intrusion of foreign matters to the inside of the rear cover. In addition, another attempt has been made to employ a brush unit with a peripheral structure in which a brush holder has a large opening oriented downward as disclosed in, for instance, U.S. Pat. No. 6,710,499. Such a structure enables the brushes, held in sliding contact with the slip rings, to be cooled, while discharging the brush-wear particles caused by the movements of the brushes in sliding contact with the slip rings.
Meanwhile, with the vehicle alternator of the related art disclosed in such U.S. patent, air streams, flowing through areas around the brushes, are created only by the rotations of the slip rings formed in cylindrical configurations. Therefore, an air capacity decreases with a degraded effect of cooling the brushes. As the brushes increase in temperature in the absence of the adequate cooling effect, the brushes undergo abnormal wear. If the abnormal wear of the brushes occurs, the discharging of the brush-wear particles is inadequately performed. This causes a short-circuiting to occur between the slip rings with a resultant halt in operation to generate electric power. Thus, an effective countermeasure is needed to address such an issue.